Synthesis and Properties of Salen–Aluminum Complexes as a Novel Class of Color‐Tunable Luminophores

Showing their true colors? Full emission color tuning in the visible region can be achieved with salen–aluminum complexes that are electronically modulated at C5 of the phenoxide ring in the salen moiety. Emission spectra for various substituents R5 are shown (EWG: electron-withdrawing group, EDG: electron-donating group). A series of salen–aluminum complexes, [{(R5)2-salen(3-tBu)2}Al(OC6H4-p-C6H5)] (salen=N,N′-bis(salicylidene)ethylenediamine; R5=H (1), tBu (2), Br (3), Ph (4), OMe (5), NMe2 (6)) and [{5,5′-(NMe3)2-salen(3-tBu)2}Al(OC6H4-p-C6H5)][OTf]2 (7; OTf=CF3SO3) that are electronically modulated directly at C5 of the phenoxide ring in the salen moiety has been prepared. The crystal structures of 1, 4, 6, and 7 determined by X-ray diffraction reveal distorted square-pyramidal geometries around the Al atoms. Complexes 1–7 are all air-stable in both the solid and solution states and have high thermal stability (decomp 313–338 °C). Differential scanning calorimetric analyses show that they can form amorphous glasses with glass transition temperatures of 95–132 °C depending on the C5 substituent. UV/Vis absorption spectra of the complexes exhibit major bands at λ=338–413 nm assignable to salen-centered π–π* transitions with a gradual red shift of the absorption maximum wavelengths as the substituent is varied from an electron-withdrawing (NMe3) to an electron-donating group (NMe2). The maxima in the emission spectra of 1–7 occur over the entire visible region, ranging from λ=438 nm for 7 to λ=599 nm for 6, with high fluorescence quantum efficiencies of up to Φ=0.40 for 4 in solution. DFT calculations suggest that the low-energy electronic transitions in 1–7 are characterized by HOMO−i–LUMO+1 (i=1 for 1–6 or i=4 for 7) transitions localized on the salen moiety, with much involvement of the C5 position in the HOMO−i. Thus, the electronic alteration at the C5 position of the phenoxide ring, which mainly affects the HOMO−i energy levels of salen–Al luminophores, is responsible for the observed emission color-tuning properties over the entire visible region.